This disclosure generally relates to a method for the purification of dihydric phenols. More particularly the disclosure relates to a method for the purification of methyl hydroquinone.
The present method of preparing dihydric phenols involves oxidation of the corresponding aromatic amines to benzoquinones followed by the reduction of the benzoquinones to hydroquinones. For example, the commercial process for the preparation of methyl hydroquinone employs o-toluidine as the raw material. The acid sulfate of o-toluidine is prepared by sulfuric acid treatment and is oxidized with manganese dioxide and sulfuric acid at lower temperatures of about 5° C. to about 8° C. The methyl benzoquinone formed is then steam distilled and reduced in the presence of zinc and/or iron and acid to form methyl hydroquinone. The isolated methyl hydroquinone has a typical purity of 99 percent and typically contains about 30 parts per million (ppm) to about 50 ppm of metals like iron, manganese, sodium, zinc, calcium and others as impurities. One of the main uses of methyl hydroquinone is in the preparation of co-polymers having good chemical resistance properties.
Methyl hydroquinone can be used in the preparation of co-polymers such as polycarbonates and polyesters. The presence of the metal residues in methyl hydroquinone in ppm levels considerably affects the properties of the co-polymer, such as for example, a reduced molecular weight buildup, reduced transparency, and an increase in color. This is true especially for dihydroxy based co-polymers and especially BPA containing PC. It is believed that metal ions present even in ppm levels, especially the transition metals like iron, can give rise to color formation during polymerization and further processing at high temperatures by forming colored metal complexes as by-products. Further, side reactions such as Fries rearrangement, which is known to be catalyzed by metals, can also occur during the polymerization.
Hence there is a need for a better purification technique that will help to reduce the metal ion concentration in the dihydric phenol to an amount such that the residual metal ion or ions will not interfere with the properties of the co-polymer prepared using the purified dihydric phenol.